Saharan input of phosphate to the oligotrophic water of the open western Mediterranean sea

A Saharan soil, considered as a proxy for Saharan aerosols, was used to perform a series of dissolution experiments: various amounts of Saharan soil were exposed to ultrapure water and seawater for varying lengths of time. The concentration of phosphate released was proportional to the amount of dust introduced. In the case of Saharan events associated with a significant amount of rain, the main dissolution of phosphorus will occur in the air column; for Saharan events associated with only few drops of rainwater, the main dissolution will occur in the surface seawater. Saharan dust represents a source of phosphate to the surface water and may play a role in biological activity, especially during the oligotrophic period. In the western Mediterranean in oligotrophic conditions, biological production is P-limited, and the atmosphere becomes the main pathway of nutrients to the surface mixed layer. At the scale of the oligotrophic season, the input of "Saharan dissolved inorganic phosporus (DIP)" are negligible compared with the new production integrated over the productive layer. At the event timescale, the production induced by "Saharan DIP" can represent up to 15% of the integrated new production and up to 14% of the total primary production in the mixed surface layer. These inputs of atmospheric DIP would promote fixation of atmospheric N₂ that, in return, may enhance the new production in the surface layer.